An axial flow, gas turbine engine has a compression section, a combustion section and a turbine section. An annular flow path for working medium gases extends axially through these sections. As the working medium gases are flowed along the flow path, the gases are pressurized in the compression sections and burned with fuel in the combustion section to add energy to the gases. The hot, high pressure gases are expanded through the turbine section to produce useful work. The work is used, for example, to power an aircraft with thrust, or to power a free turbine with pressurized gases and to pressurize the gases in the compression section.
A rotor assembly extends axially through the engine to transfer the work of pressurization from the turbine section to the compression section. In the turbine section, the rotor assembly includes rotor disks each having arrays of rotor blades which extend outwardly across the working medium flow path. The arrays of rotor blades are angled with respect to the approaching flow to receive work from the gases and to drive the disks about the axis of rotation.
A stator assembly circumscribes the rotor assembly. The stator assembly has an outer case which contains the working medium gases and arrays of stator vanes which are attached to the outer case. Each array of vanes extends inwardly across the working medium flow path upstream of an associated array of rotor blades. The stator vanes direct the working medium gases into the arrays of rotor blades at angles which optimize the performance of the engine. Accordingly, it is important that the working medium gases flow through the stator vanes and not around the tips of the stator vanes to preserve the efficiency of the engine.
However, the stator vanes are spaced radially from the rotor assembly by a clearance gap during normal operation to avoid contact between the blades and the vanes which might destroy either the moving blades or the stationary vanes. A labyrinth seal is provided to block the gases from flowing through this clearance and around the vanes. The labyrinth seal is formed of a seal land on the stator vanes and knife edge elements on the rotor assembly. Examples of such constructions are shown in U.S. Pat. No. 3,826,084 entitled "Turbine Cooling Flow System" issued to Branstorm et al. and U.S. Pat. No. 4,213,738 entitled "Cooling Air Control Valve" issued to Williams. In these constructions, an inner air seal carrying the knife edges extends between adjacent rotor disks.
The maximum size of the gap is set by the transient growth which occurs during an increase in an engine power from low power to high power. As the engine is accelerated from a low power such as idle power to a high power such as sea level takeoff power, the temperature of the working medium gases increases dramatically. The stator vanes are in intimate contact with the hot working medium gases and are rapidly heated as a result of the increased temperature of the gases. The outer case which is more remotely located from the working medium flow path responds more slowly to the gases than do the vanes. The vanes expand inwardly toward the rotor assembly decreasing the clearance between the seal land on the vanes and the knife edges on the inner air seal of the rotor assembly.
Movement of the inner air seal outwardly toward the stator vanes during acceleration of the engine increases the size of the gap needed to accommodate transient operation. The amount of thermal growth experienced by the seal is increased by locating the inner air seal in close proximity to the working medium flow path because the seal is rapidly heated by the working medium gases. As a result, the gap between the array of stator vanes and inner seal must be increased even further to insure that destructive interference between these components does not occur during transient operation of the engine.
One possible way of decreasing the gap by decreasing the outward movement of the air seal is shown in U.S. Pat. No. 3,343,806 issued to Bobo et al. entitled "Rotor Assembly For Gas Turbine Engines." Bobo discloses an inner air seal mounted on a rotor disk. The rotor disk extends radially between the blade carrying disks and is described as being relatively massive. The rotor disk acts to radially support the inner air seal to decrease stresses in the seal and to act as a heat sink for conducting heat away from the knife edge elements.
In modern engines, it is also desirable to decrease the mass of the inner air seal to minimize the adverse impact that the mass of the seal has on engine performance. Accordingly, scientists and engineers are working to develop an inner air seal extending between rotor disks which is positively restrained against thermal expansion and which does not have a relatively large mass in comparison with inner air seals which are not similarly restrained.